1. Field of the Invention
The present invention relates in general to an adjus compensation device for fiber Bragg gratings.
2. Description of the Related Art
Fiber Bragg gratings (FBGs) are very important elements widely used in the fabrication of various functional devices for dense WDM networks, for example FBG stabilized laser sources and various FBG-based WDM devices for multiplexers, demultiplexers and add/drop filters. In the practical application of FBGs, however, a problem arising from changes in the surrounding temperature has been noticed. Because the pitch of the FBGs determines the central frequency of the reflected optical signal transmitted in an optical fiber, the FBGs are carefully designed and accurately manufactured. The problem is that the optical fibers elongate in a raised surrounding temperature so that the pitches of the FBGs deviate from the design value. Such a situation is not desirable.
FIG. 1A shows a temperature compensation device for fiber Bragg gratings using a bi-metal structure which includes two arms 13, 13' and two metal plates 14, 15. The two metal plates 14, 15 are welded together while the two arms 13, 13' are welded at the sides of the metal plates 14, 15. The thermal expansion coefficient of the metal plate 14 is smaller than that of the metal plate 15.
In operation, an optical fiber 11 provided with FBGs 12 are glued onto the arms 13, 13'. The optical fiber 11 tends to elongate in a raised surrounding temperature. However, the temperature compensation device bends, as shown in FIG. 1B, due to different thermal expansion coefficients between the two metal plates 14, 15, so as to prevent the optical fiber 11 from elongating. By this arrangement, the change in the pitches of the FBGs 12 arising from the raised temperature can be greatly reduced.
The above-mentioned temperature compensation device for fiber Bragg gratings does lessen the influence of temperature change on the FBGs. However, the compensation in such a way is not very accurate because of the tolerances arising from manufacturing and packaging (In operations, the temperature compensation device is packaged by a box with the ends of the optical fiber fixed to the box. This can prevent an external force, such as a careless pull at the optical fiber, from destroying the temperature compensation device). For example, the tolerances may arise from over-stretching the optical fiber when: (1) gluing the optical fiber onto the bi-metal structure; (2) attaching the ends of the optical fiber to the box for packaging. It is understood that tolerances are not considered in design. However, tolerances do influence the control over the changes of the length of the optical fiber so that the compensation of the change of pitches of the FBGs is not accurate.